1. Technical Field
The present invention relates to a load control device and more specifically to a lighting dimmer that is operable when connected either in parallel or in series with an electrical load and is further adaptable as a replacement for a three-way switch.
2. Background Art
It is known to those skilled in the art, that standard 120 VAC residential wiring provides three conductors (e.g. wires) to which an installer can attach various equipment, such as lighting dimmers. It is also known that two of these three conductors are active current carrying conductors, meaning that connections to these active conductors are required to complete a functional circuit. The US National Electrical Code specifies, for 120 VAC residential wiring, that a first active conductor with significant voltage to earth ground be identified as a ‘hot’ (H) conductor and color-coded black and that a second active conductor near ground potential be identified as a ‘neutral’ (N) conductor and color-coded white. US National Electrical Code further specifies, for 120 VAC residential wiring, a third (passive) safety ground (G) conductor not normally used to carry circuit current and color-coded as green or as a bare uninsulated conductor.
Dimmers are devices used to vary the brightness of a lamp typically by modulating the duty cycle of the supplied AC power, such as by controlling a triac to turn on only after waiting for a period time has elapsed since a zero-crossing and allowing the triac to turn off at the next zero crossing. Although such phase control devices can be used for various purposes, the term dimmer is generally reserved for those intended to control lighting. By decreasing or increasing the AC duty cycle and hence the mean power to the lamp it is possible to vary the intensity of the light output from the lamp. Such a variable duty cycle AC voltage output is controlled by a dimmer, referenced to neutral (N), and is identified as a ‘dimmer hot’ (DH) active conductor.
There are two ways to power lighting dimmers designed as replacements for standard two-way switches that are known in the prior art. Prior art FIG. 1 illustrates a dimmer with one active input wire and a prior art FIG. 2 illustrates a dimmer with two active input wires. As shown in prior art FIG. 1, the dimmer with one active input wire 12 is installed such that its internal power supply is sourced in series with the lighting load 13 to be dimmed, such as for example, an incandescent lamp. For descriptive purposes herein, dimmers with one active input wire are also referred to as serially installed dimmers, serially sourced dimmers or some variation thereof. As shown in prior art FIG. 2, the dimmer with two active input wires 22 is installed such that its internal power supply is sourced directly from the hot (H) and neutral (N) line voltages of the AC voltage source 16. Dimmers with two active input wires are hereafter also referred to as parallel installed dimmers, parallel sourced dimmers or some similar variation thereof. Dimmers sourced in series with the lighting load 12 are more prevalent in renovation work because they can be installed as direct replacements for standard wall switches without modifying the associated residential house wiring. Dimmers sourced in parallel 22 are typically used in new construction and in electrical upgrades where a neutral wire is available.
Similarly, there are two ways to power lighting dimmers that are designed to replace three-way switches. Prior art FIG. 3 illustrates a dimmer with one active input wire and two active output wires designed for use in place of a three-way switch, as disclosed in Blakely (U.S. Pat. App. 2009/0039854). Prior art FIG. 4 illustrates a dimmer with two active input wires and two active output wires designed for use in place of a three-way switch, also disclosed in Blakely. It is important to note that the prior art does not recognize that combined serial/parallel implementation can be achieved.
Standard on-off switches, known in the industry as two-way switches or more formally as single-pole single-throw switches (SPST), are the simplest type of switch employed in lighting circuits and allow for a modest level of load control. Three-way switches 33, commonly implemented as single-pole double-throw (SPDT) switches, are employed in more complex wiring schemes where it is desirable to have more than one control point for the external load 13. For example, a pair of three-way switches wired in series with a load provides two control points. It is known to those skilled in the art that by inserting one or more four-way switches, commonly implemented as double-pole double-throw (DPDT) switches, between a pair of three-way switches the number of control points for the circuit can be extended indefinitely. Each four-way switch adds one additional location where the load can be controlled. If a neutral (N) wire 25 is available in the wall box, the parallel sourced dimmer with two active output wires 42 shown in prior art FIG. 4 can replace a single three-way switch. If no neutral (N) wire 25 is available in the wall box, the serially sourced dimmer with two active output wires 32 shown in prior art FIG. 3 can replace a single three way switch.
Prior art FIG. 1 shows common residential wiring practice when a dimmer replaces an existing two-way switch when there is no neutral (N) wire 25 present in the residential wall box. In this case, a dimmer 12 is serially connected between the AC source 16 hot (H) conductor and the lighting load 13 hot (H) conductor. The dimmer 12 hot (H) conductor connects to the AC power source 16 hot (H) conductor and the dimmer hot (DH) conductor from the serially installed dimmer 12 connects to the hot conductor (h) of the lighting load 13. In this wiring scheme 10, the serially sourced internal power supply 12 can only charge when there is a voltage across the dimmer 12. For example, when a switching device internal to the serially installed dimmer 12, such as an internal triac, is not conducting a voltage will be present across the dimmer 12 and the internal power supply can charge.
The charging path for the serially sourced dimmer 12 power supply is from AC source 16 hot, the dimmer 12 exiting as dimmed hot (DH), through the lighting load 13, and then returning to AC source 16 neutral (N). Therefore, to charge its internal power supply, the dimmer 12 internal switching device must be non-conducting for a portion of each AC half cycle. This limits the maximum lighting level of the light load 13 and also limits the maximum load resistance that can be connected. Typically, connecting a lighting load 13 less than 50 watts at 120 VAC causes problems when charging the serially sourced dimmer 12 power supply at high end (e.g. maximum light level). This problem is exacerbated for dimmers with one active input wire that require additional current draw, such as those containing radio transceivers or other components.
Refer now to prior art FIG. 2. When a neutral (N) wire 25 is present in a wall box, such as in new construction, it is desirable to connect a dimmer with two active input wires 22 between the AC source 16 hot (H) and neutral (N) conductors and the lighting load 13 because the dimmer internal power supply is always connected to a power source. Such a neutral wire 25 can be provided at the wall box by inserting a splice 24 into the circuit neutral wiring to the lighting load 13. Variable voltage, with respect to circuit neutral (N) is provided from the three-active-wire dimmer 22 as a dimmed hot (DH) output to the lighting load 13 hot (H) conductor. Advantageously, in this configuration 20 a power supply, internal to the three-active-wire dimmer 22, will always be able to charge up through the hot (H) to neutral (N) path thus eliminating constraints on high end (maximum light output) and minimum load wattage. The neutral (N) wire 25 connection also provides a means to obtain an accurate zero crossing signal from the AC source 16. The neutral (N) wire 25 connection also allows a parallel installed dimmer 22 to operate as a full-on/full-off electronic switch since the output can go to full conduction. Such a full-on/full-off mode is desirable if the lighting load is replaced with a general appliance, such as an electric fan.
Refer now to prior art FIG. 3. In addition to the AC source 16 hot (H) connection, the prior art lighting dimmer 32 has two dimmer hot (DH1, DH2) connections which are each connected to a corresponding pole of a three-way switch 33. Advantageously, the addition of the second dimmer hot (DH2) connection and its corresponding control circuitry provides two possible conductive paths 321, 322 from the AC power source 16 to the lighting load 13, each corresponding to a three-way switch 33 position. It should be noted that in this circuit 30 the internal power supply is sourced in series with the load 13. Therefore, those skilled in the art will recognize that the same disadvantages of the serially sourced dimmer 11 shown in prior art FIG. 1, limitations on maximum lighting level and maximum load resistance, apply to this dimmer 32 as well.
Prior art FIG. 4 illustrates a residential lighting circuit 40 with a parallel sourced lighting dimmer 42 coupled to a three-way switch 33. In addition to the AC source hot (H) connection and the neutral (N) connection, the dimmer 42 has two dimmer hot (DH1, DH2) connections which are each connected to a corresponding pole of the three-way switch 33. Just as in the prior art dimmer shown in FIG. 3, the addition of the second dimmer hot (DH2) connection and its corresponding control circuitry provides two possible conductive paths 421, 422 from the AC power source 16 to the lighting load 13, each corresponding to a position of the three-way switch. However, because there are two active input wires, the internal power supply is always able to charge up through the hot (H) to neutral (N) path thus eliminating constraints associated with serially installed dimmers.
One of the specific problems in the field is that each of these dimmers is designed for use in one respective wiring configuration and is not easily configurable for use in another configuration. For instance, a prior art dimmer with one active input and one active output is a suitable replacement for an existing two-way switch but cannot be used in place of a three-way switch and is not desirable in new constructions where parallel sourced dimmers may be installed. Installers must suffer the cost and complexity associated with stocking multiple dimmer types.
In response to this problem, certain manufacturers have developed manually configurable dimmers with a neutral (N) wire connection that can either be connected to AC source neutral (N) (providing two active input wires) or externally connected to the lighting load along with the dimmed hot (DH) wire (providing one active input wire). When these manually configurable dimmers operate with one active input wire, the neutral (N) wire needs to be connected to the lighting load hot (H) wire along with the dimmed hot (DH) wire to ensure a symmetrical flow of current through the lighting load when the internal power supply is charging. This is necessary especially for magnetic load types, such as low voltage transformers, because charging up asymmetrically can lead to heating in the transformer coil which can damage the transformer. Charging asymmetrically also limits the ability for the manually configurable dimmer internal power supply to charge fully.
During installation, those skilled in the art will recognize that a manually configurable dimmer can be set to operate in the above described serial configuration by adding an external jumper between the neutral (N) and dimmer hot (DH) terminals. Typically, this additional external jumper connection is simple to make as flying wire leads from the dimmer can be connected appropriately using wire nuts. However, it is still desirous for lighting dimmers to employ screw terminals similar to those found on standard light switches, such as a light switch being replaced by said dimmer, to make installation easier. In such an easier installation, the dimmer would be a drop-in replacement for the switch being removed and would not require bulky wire nuts to be squeezed into the residential wall box. These desired screw terminals pose a problem with making the external jumper connection between the neutral (N) terminal and the dimmed hot (DH) terminal of the manually configurable dimmer known in the prior art. The external jumper would still be needed to configure the manually configurable dimmer into a two-wire mode as in the equivalent dimmer with flying leads but when screw terminals are introduced, this external jumper is likely to be ignored or misapplied by the installer resulting in faulty operation or failure of the device.